Remove unused write function

Bug: n/a
Test: m
Change-Id: I393abe1436f551f949259b8a6fd07bec0c01305b
diff --git a/compositediskconfig/src/lib.rs b/compositediskconfig/src/lib.rs
index 3546dd3..dc199e4 100644
--- a/compositediskconfig/src/lib.rs
+++ b/compositediskconfig/src/lib.rs
@@ -15,7 +15,6 @@
 //! JSON configuration for composite disks, as used for running `mk_cdisk` and by the `vm` tool.
 
 use serde::{Deserialize, Serialize};
-use std::io::Write;
 use std::path::PathBuf;
 
 /// Configuration for running `mk_cdisk`.
@@ -36,10 +35,3 @@
     #[serde(default)]
     pub writable: bool,
 }
-
-impl Config {
-    /// Write the configuration as JSON, in the format used by `mk_cdisk`.
-    pub fn write_json(&self, writer: impl Write) -> serde_json::Result<()> {
-        serde_json::to_writer(writer, self)
-    }
-}